I should have said "men" and not just "gay men" and well, pretty much just include most humans in that too, but men mostly due to their uncanny ability to get hard when even just a light wind touches their penis.
I am not sure if the hard on is from a response of the body though, especially after a hand job or a massage on the prostate. I remember reading about some women raping men and the men were questioned how they were hard. But i agree with you. The limp dick is so rare in rape manga that is getting ridiculous! ╮(￣▽￣)╭
I think women raping men and men raping men are different. A straight man getting a hand job from a relatively normal looking woman, even if it was forced and against his will, is on a different level. The mind is saying stop, this is wrong, but the body says, hey feels good. If the victim thought the woman was ugly, and she had him tied up and was beating the shit out of him, he would have to be a serious M to get an erection. And of course there's all sorts of other factors, are they gay, straight, higher or lower emotional levels, pain tolerance/intolerance, personal mind set, and tons more.
As far as rape scenes go, at least this one had a proper response from Ayumu - a limp dick. If it was a dick re-drawer or the mangaka, either way, kudos. I don't like rape scenes, but I don't make a fuss about them (this is yaoi, pass if you must), and I think it's so stupid to show an uke getting raped with a raging hard on. Come on, gay men are horny and love sex, sure, but that doesn't mean they'll get an erection for any kind of sex. Violence, pain, and fear are excellent libido killers.